An ignition plug is used in an internal combustion engine or the like, and includes an insulator formed of an insulating ceramic such as alumina and having an axial bore extending in the axial direction thereof. A center electrode is provided in a forward end portion of the axial bore. A terminal electrode is provided in a rear end portion of the axial bore, and a ground electrode which forms a spark discharge gap in cooperation with the center electrode. Spark discharge is generated at the spark discharge gap by applying a predetermined voltage to the spark discharge gap through the terminal electrode.
The terminal electrode includes a head portion which projects from the rear end of the insulator and to which a high tension cable (plug cap) for power supply is attached, and a rod-shaped leg portion which is inserted into the axial bore and whose forward end portion is fixed to the insulator by means of glass seal or the like. The head portion is provided in a state in which a flat-shaped portion of the head portion is in contact with a flat-shaped portion (flat portion) of a rear end surface of the insulator (in some cases, at least a portion of the forward end surface of the head portion may separate from the rear end surface of the insulator).
In recent years, the fuel consumption of an internal combustion engine has been strictly regulated from the viewpoint of environmental protection, etc. In order to prevent a decrease in the output of the internal combustion engine while complying with the regulation on fuel consumption, the displacement of the internal combustion engine has been decreased. A decrease in the output of the engine has been prevented by increasing the degree of compression and the degree of supercharging of the engine.
In an internal combustion engine having an increased degree of compression and an increased degree of supercharging, a higher voltage is needed so as to generate spark discharge. However, when the applied voltage is increased, current may leak from the terminal electrode to the metallic shell while creeping along the surface of the insulator, and misfire may occur due to discharge anomaly. In view of this, there has been proposed increasing the length of a portion (rear trunk portion) of the insulator located between the rear end of the metallic shell and the head portion of the terminal electrode to thereby prevent leakage of current (so-called flashover) (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-155839).
Moreover, in recent years, an ignition plug has been demanded to have a reduced size; in particular, a reduced diameter, and the insulator thereof may have a reduced diameter in order to meet such demand. In the case of the insulator having a reduced diameter, the wall thickness of the rear end portion is decreased, and the area of the above-mentioned flat portion of the insulator is decreased.
Incidentally, when a vibration generated as a result of operation of an internal combustion engine or the like acts on an ignition plug used therein, the head portion of the terminal electrode shakes, with a forward end portion of the leg potion fixed to the insulator serving as the center of the shaking motion, whereby the leg portion may come into contact with the inner circumference surface of a rear end portion of the insulator. In the case where the wall thickness of the rear end portion of the insulator is small, breakage such as cracking may occur on the inner circumferential surface of the rear end portion of the insulator as a result of the contact with the leg portion.
Also, the vibration applies a load onto the head portion of the terminal electrode in a direction intersecting with the axis, and due to this load, the forward end surface of the head portion may be pressed against the flat portion of the insulator. When the forward end surface of the head portion is pressed against (comes into pressure contact with) the insulator, a compressive load acts on a portion of the insulator against which the head portion is pressed, and a tensile load acts on a portion of the insulator adjacent to the portion on which the compressive load acts. Since the insulator is strong against compressive load but relatively weak against tensile load due to its nature, the above-mentioned tensile load may cause breakage at the rear end surface (flat portion) of the insulator.
In particular, in the case of an insulator in which the length of its rear trunk portion is increased and/or the area of the above-mentioned flat portion of the insulator is decreased, breakage is more likely to occur.
The present invention has been conceived in view of the above circumstances. An advantage of the invention is an ignition plug which can prevent breakage at a rear end portion of an insulator even in the case where a rear trunk portion of the insulator is long and/or the area of a flat portion of the insulator is small.